The present invention relates to a predictive decoder arranged on a side opposite to a predictive coder for performing predictive coding of a motion image signal and, more particularly, to a predictive decoder used for a predictive coder for changing a quantization characteristic in accordance with the data amount of a coded signal.
In a conventional predictive coder of this type, control is performed to change a quantization characteristic in accordance with the data amount of a coded signal as an output.
In this conventional predictive coder, with an increase in data amount of a coded signal, a coarser quantization characteristic is selected. As a result, when a coded signal is decoded by a predictive decoder, a deterioration in image quality, e.g., false contouring (outlines, like contours of a map, appearing at a portion, on a screen, where luminance is gradually changed when the quantum step size is large) occur.